Steven Moffat has hinted that the main mystery so far in the new season of Doctor Who will be solved sooner than you might think.

The Doctor and Nardole have been seen guarding a mysterious vault since Season 10’s opener, ‘The Pilot’, last month.

Teasing the upcoming Episode 6, which he has written, Moffat told Doctor Who Magazine: “It’s the absolute classic mystery, isn’t it? It’s the locked door. It’s the simplest thing I could do – the Doctor’s got a locked door – but it’s instantly fascinating. I mean, you’d only have to be staying in someone’s house for a while and realise that one of the doors is locked to become impossibly curious about what’s in there. That’s human nature.”

The showrunner added: There is something perfectly fascinating about a locked door. And we don’t have to wait a whole 12 episodes to find out what’s in there, which I think is important, or you’d go mad.”

Episode 6, titled ‘Extremis’, will air on Saturday 20 May.

The official synopsis reads: “In the Haereticum – the Vatican’s secret library of blasphemy – there is an ancient book known only as The Veritas. Throughout history, anyone who has ever read it has immediately taken their own life.

“Now a new translation is online, and the danger is spreading. The Vatican appeals to the Doctor. Will he read The Veritas? But can even the Doctor survive the ultimate truth?”

Season 10 continues at 7.20pm this Saturday on BBC One with ‘Knock Knock’.

Pre-order the new season on DVD on Amazon here.

Pre-order the new season on Blu-ray on Amazon here.

Issue 512 of Doctor Who Magazine is out now.

What do you think is in the vault? Let us know below…